1. Field of the Invention
The present invention relates to a chlorosilane disproportionation catalyst and a method for producing a silane compound by means of the catalyst. More particularly, the present invention relates to a chlorosilane disproportionation catalyst comprising a specific tertiary amine and a hydrochloride thereof as the main components, and a method for continuously producing a silane compound such as dichlorosilane, monochlorosilane or monosilane, by supplying a starting material chlorosilane and the catalyst into a reaction tower, whereby the disproportionation reaction by means of the specific catalyst and the separation by distillation are conducted simultaneously. Further, the present invention relates to a process for producing a silane compound efficiently by combining a process for synthesizing trichlorosilane from silicon of metallurgical grade or silicon tetrachloride, with the above method for producing a silane compound.
2. Description of the Prior Art
Demands for silane compounds such as dichlorosilane (SiH.sub.2 Cl.sub.2), monochlorosilane (SiH.sub.3 Cl) and monosilane (SiH.sub.4) are expected to increase more and more as they are useful as the raw materials for the high purity silicon to be used for elements for semiconductors, solar cells, etc. In particular, it has been desired that dichlorosilane and monosilane be produced efficiently and in a large quantity.
It is known to obtain a silane by the disproportionation reaction of SiHCl.sub.3 in the presence of a catalyst, in accordance with the following equilibrium reactions. EQU 2SiHCl.sub.3 .revreaction.SiH.sub.2 Cl.sub.2 +SiCl.sub.4 ( 1) EQU 2SiH.sub.2 Cl.sub.2 .revreaction.SiHCl.sub.3 +SiH.sub.3 Cl (2) EQU 2SiH.sub.3 Cl.revreaction.SiH.sub.4 +SiH.sub.2 Cl.sub.2 ( 3) EQU 4SiHCl.sub.3 .revreaction.SiH.sub.4 +3SiCl.sub.4 (as a whole) (4)
The disproportionation of chlorosilanes has been studied since old, and various proposals have been made. However, they still have disadvantages. For example, (i) the method of using nitriles as taught in U.S. Pat. No. 2,732,282 has to be conducted at a reaction temperature of 150.degree. C. and above; (ii) the method of using aliphatic cyanamides as taught in U.S. Pat. No. 2,732,280 requires the pretreatment with a Lewis acid; (iii) the method of using dimethylformamide or dimethylbutylamide as taught in U.S. Pat. No. 3,222,511 is liable to deteriorate the catalyst used for the reaction; and (iv) the method of using a tertiary amine containing a hydrocarbon composed of an alkyl group with 1 or 2 carbon atoms, as taught in U.S. Pat. No. 2,834,648 is required to be conducted at a temperature of 150.degree. C. and above, as is the case with the catalyst of the above-mentioned method (i), besides which a pressure resistant vessel has to be used, and, in spite of its equilibrated conversion ratio (calculated value) being 18% at the reaction temperature of 150.degree. C., the actual conversion ratio is as low as about 10%, hence a large size apparatus is required to attain a desired quantity of production.
Further, U.S. Pat. No. 4,113,845 discloses use of a fixed bed type reactor packed with an anion exchange resin containing a tertiary amine as the catalyst, wherein a starting material chlorosilane such as trichlorosilane or dichlorosilane is supplied in a liquid state from one port of the reactor and reacted at a temperature of from 30.degree. to 200.degree. C. under a pressure of from 1 to 30 atm, whereby a reaction product comprising monosilane, monochlorosilane, dichlorosilane, trichlorosilane and silicon tetrachloride is obtainable from the other port of the reactor. However, since the above-mentioned equations (1), (2) and (3) for the disproportionation reaction are equilibrium reactions, it is not possible to complete the reaction of the starting material chlorosilane 100%, even if the reaction is conducted for an extended period of time. For example, the following Table indicates the equilibrated composition of monosilane, monochlorosilane, dichlorosilane, trichlorosilane and silicon tetrachloride at a temperature of 80.degree. C. when the disproportionation reaction of trichlorosilane or dichlorosilane as the starting material has reached the state of equilibrium.
______________________________________ (mole %) Starting material Mono- Tri- Silicon chloro- Mono- chloro- Dichloro- chloro- tetra- silane silane silane silane silane chloride ______________________________________ Trichloro- 0.04 0.52 10.6 77.1 11.8 silane Dichloro- 10.2 15.6 38.8 34.7 0.65 silane ______________________________________
Further, even when the disproportionation reaction is brought to the equilibrated condition by using trichlorosilane as the starting material, monosilane and monochlorosilane contained in the reaction product are 0.04 mol % and 0.52 mol %, respectively, which figures indicate a very low reaction rate for producing monosilane or monochlorosilane in a single stage reaction. Therefore, when, for example, monosilane is produced by using trichlorosilane as the starting material, the reaction product from the reaction vessel at the first stage is charged in a distilling device to separate a mixture containing therein monosilane, monochlorosilane, and dichlorosilane, wherein dichlorosilane is the principal component (composition A) and a mixture of trichlorosilane and silicon tetrachloride (composition B), and then the composition A is fed into the reaction vessel at the second stage, whereupon, since the equilibrated composition from the disproportionation reaction contains 10.2 mol % of monosilane, as is apparent from the table, monosilane can be separated and recovered by feeding this reaction product into the distilling apparatus. However, since the rate of reaction in the disproportionation reaction is low, the unreacted substance should be circulated in a large quantity for use, with the consequence that enormous amount of energy was disadvantageously required for the operations of the reaction vessel and the distilling tower.
Furthermore, N-methyl-2-pyrrolidone, methylimidazole, tetramethylurea, tetramethylguanidine, trimethylsilylimidazole, benzothiazole, N,N-dimethylacetamide and the like, as disclosed e.g. U.S. Pat. Nos. 4,018,871 and 4,038,371 or Japanese Unexamined Patent Publication No. 17918/1981, exhibit catalytic activities in the disproportionation reaction of silane compounds. However, they are per se solid, or become powdery solid when brought in contact with a silane compound such as trichlorosilane or dichlorosilane. Consequently, it becomes difficult to separate them from the silane compound produced by the disproportionation reaction. Thus, they are hardly applicable to a practical operation on an industrial scale.